1. Field of the Invention
The invention relates to a control apparatus and control method for an engine in which an output shaft is connected to an automatic transmission (or a manual transmission with an automatic clutch). More particularly, the invention relates to a control apparatus and control method which performs torque-down control during shifting in order to suppress shift shock. While the engine control apparatus and engine control method according to this invention are preferably used for controlling an engine for a vehicle such as an automobile, it is to be understood that they may also be used for controlling an engine for various kinds of machinery and apparatuses other than a vehicle.
2. Description of the Related Art
Transient sudden fluctuations in output torque of a transmission, i.e., shift shock, is known to occur when the speed of an automatic transmission that is connected to an output shaft of the engine is changed (i.e., during shifting). Simply put, shift shock is mainly produced by rotary inertia torque of the engine when the engine speed changes are added on to the output torque of the engine (i.e., torque generated by the combustion of fuel) and the resultant torque is then transmitted to an input shaft of the transmission during the shifting of speeds. One idea that has been proposed to avoid shift shock is to cancel out the effects of engine rotary inertia by temporarily reducing the output torque from the engine in response to a request by the driver to shift speeds or an automatic request to shift speeds from a transmission control apparatus. Control which reduces the output torque from the engine in this way is referred to as “torque-down control during shifting”. Reducing the output torque from the engine is performed, for example, by delaying (i.e., retarding) the ignition timing of the engine (see Japanese Patent Publication No. 4-67058, Japanese Patent Application Publication No. JP-A-7-293682, and Japanese Patent Application Publication No. JP-A-2002-188476), or by reducing the opening amount of the throttle (see Japanese Patent Application Publication No. JP-A-9-125998).
When performing torque-down control by retarding the ignition timing, it is known that if an MBT (Minimum advance for Best Torque) ignition timing of the engine (i.e., the ignition timing that generates the most torque in the current engine operating state) is determined, then a torque-down rate (i.e., the ratio of torque reduced after the retard to the torque generated in the engine at the MBT ignition timing (i.e., the maximum amount of torque able to be generated)) is primarily determined by the retard amount measured from the MBT ignition timing, regardless of the other operating states (see FIG. 4A), as described in Japanese Patent Application Publication No. JP-A-2002-188476. Accordingly, if the MBT ignition timing and the required torque-down rate (or target torque-down rate) are provided, the retard amount that will achieve that required torque-down rate can be obtained, which enables the target ignition timing to be determined. During actual operation of an engine, the MBT ignition timing changes depending on the operating state of the engine. Therefore, during this control, the retard amounts that provide the various torque-down rates when there are various MBT ignition timings (crank angles) are obtained through experimentation beforehand, and a two-dimensional data map or table of retard amounts is prepared in advance with the MBT ignition timing and the torque-down rate as parameters. The target retard amount that provides the required torque-down rate (i.e., the torque-down rate necessary to avoid shift shock) is determined using this two-dimensional data table that was prepared in advance (the target ignition timing is set to a timing that corresponds to the target retard amount). That is, even if the operating state of the engine changes, as long as the MBT ignition timing for the current engine operating state is provided, then the target retard amount for achieving the required torque-down rate, i.e., the appropriate ignition timing, can be determined from the relationship between the torque-down rate and the retard amount obtained beforehand through experimentation, such that the amount of torque output from the engine can be precisely controlled.
Japanese Patent Application Publication No. JP-A-2002-188476 also points out that torque lost due to friction between structural elements of the engine (i.e., friction torque) should be taken into account in order to more precisely execute torque-down control. When it is necessary to adjust the torque transmitted from the output shaft of the engine to the transmission (i.e., the net engine torque) in order to avoid shift shock, the torque generated by the combustion of fuel in the engine can be adjusted by controlling the ignition timing or the opening amount of the throttle. In an actual engine, a portion of all of the rotary energy generated by the combustion of fuel is lost due to friction between structural elements of the engine such that the net engine torque is less than the torque generated by the combustion of fuel. As a result, in this control, the required torque-down rate or the target torque-down rate is determined taking that friction torque amount into account. One example of a method for calculating the friction torque of the engine disclosed in Japanese Patent Application Publication No. JP-A-2003-120801 estimates the friction torque from the engine intake air pressure and the engine speed. Another method disclosed in Japanese Patent Application Publication No. JP-A-2005-30252 estimates the value of the friction torque from the engine coolant temperature. Furthermore, relating to torque-down control of an engine during shifting, Japanese Patent Application Publication No. JP-A-2004-84820 describes technology in which shift control of the transmission itself is performed be referencing the difference between the actual engine output torque and a target value of that engine output torque. Japanese Patent Application Publication No. JP-A-2005-30252 also describes technology which performs shift control using the hydraulic pressure and temperature of the hydraulic fluid in a transmission to calculate the torque component due to friction between structural elements in a transmission.
When performing torque-down control by retarding the ignition timing, the torque-down rate when the MBT ignition timing is at a given crank angle is primarily determined by the retard amount, as described above. Therefore, the target retard amount is determined using a two-dimensional data table which has limited retard amount data obtained through experimentation beforehand with the MBT ignition timing and the required torque-down rate as two variables, as shown in FIG. 4B. In this two-dimensional data table, a state specified at predetermined intervals of time, i.e., data (the retard amount) at a data point, is provided by the two variables (i.e., the MBT ignition timing and the required torque-down rate), as is well known. During actual control, if the values of the current MBT ignition timing and the required torque-down rate do not match the respective variable values for which the retard amount data is obtained in advance (which is often the case), i.e., if data point a in the two-dimensional data table specified by the actual MBT ignition timing and the required torque-down rate does not match any of the data points b0 to b3 for which the retard amount data is obtained in advance, as shown in FIG. 4C, then either the retard amount data at the closest data point provided in the table (such as b0) is selected or the target retard amount is determined by interpolation using the retard amount data at a plurality of lattice points b0 to b3 that are close.
In order to accurately determine the target retard amount, however, as shown in FIG. 4B, the amount of retard amount data to be stored in the two-dimensional data table of retard amounts with the MBT ignition timing and the required torque-down rate as the variables is enormous. And because of this, an enormous amount of labor would be spent on experimentation in order to prepare the table. If the retard amount data of the data point that is closest to the data point specified by the current MBT ignition timing and the required torque-down rate is made the target data mount, an extremely large amount of retard amount data is necessary to reduce the error. Further, even if the target value is determined by interpolating the retard amount data, the change in the retard amount that provides a given required torque-down rate with respect to the change in the MBT ignition timing is non-linear, as shown in FIG. 5A (i.e., the retard amount changes up and down). Therefore, in order to accurately determine the target retard amount by linear interpolation of the retard amount data, it is necessary to obtain detailed retard amount data for the MBT ignition timing on a level at which changes in the retard amount data between adjacent data points are considered substantially linear when preparing the table. That is, in this case as well, it is necessary to obtain an extremely large amount of retard amount data through experimentation beforehand. As the amount of data that needs to be obtained through experimentation in advance increases, so too does the cost of memory to store that data, or the cost of configuring the memory, including the expenses for the experimentation to obtain the data.
Also, as described above, while performing torque-down control during shifting, the ignition timing and the intake air amount and the like change, which result in a fluctuation in other operating states of the engine such as the engine speed, the MBT ignition timing, the air-fuel ratio, the engine exhaust gas temperature and the like. In order to control the engine output torque more accurately and appropriately, it is preferable that the engine control apparatus also be able to appropriately (more appropriately than the apparatuses disclosed in the related art described above) handle changes in the engine operating states during torque-down control.